1. Field of the Invention
The invention relates to a plasma display panel, and more particularly to a plasma display panel which is capable of preventing display defectiveness caused by breakage and/or defective shape of partition walls.
2. Description of the Related Art
A plasma display panel is recently often used as a flat display, because a plasma display panel has advantages that it is thin and can be readily applied to a big screen, it has a broad viewing angle, and it has a high response speed.
FIG. 1 is a perspective view of a display cell in a conventional three-electrode surface-discharge AC type plasma display panel.
As illustrated in FIG. 1, a front substrate 351 and a rear substrate 352 are arranged parallel to each other in a display cell.
The front substrate 351 is comprised of an electrically insulating substrate 302 composed of transparent material such as glass, a plurality of scanning electrodes 303 (only one of them is illustrated in FIG. 1) formed on the substrate 302 in facing relation to the rear substrate 352, a plurality of common electrodes 304 (only one of them is illustrated in FIG. 1) formed on the substrate 302 in facing relation to the rear substrate 352, a plurality of trace electrodes 305 each formed on each of the scanning electrodes 303, a plurality of trace electrodes 306 each formed on each of the common electrodes 304, a dielectric layer 312 formed on the substrate 302, covering the scanning electrodes 303, the common electrodes 304 and the trace electrodes 305 and 306 therewith, and a protection layer 313 formed on the dielectric layer 312.
The scanning electrodes 303 and the common electrodes 304 are arranged alternately, and equally spaced away from adjacent ones in parallel with one another.
The trace electrodes 305 and 306 reduce an electrical resistance of the scanning electrode 303 and the common electrode 304, respectively.
The protection layer 313 protects the dielectric layer 312 from discharges. The protection layer 313 is composed of magnesium oxide (MgO), for instance.
The rear substrate 352 is comprised of an electrically insulating substrate 301 composed of transparent material such as glass, a plurality of data electrodes 307 formed on the substrate 301 in a direction perpendicular to a direction in which the scanning electrodes 303 and the common electrodes 304 extend, in facing relation to the front substrate 351, a dielectric layer 341 formed on the substrate 301, covering the data electrodes 307 therewith, a partition wall 315 formed on the dielectric layer 314, and a phosphor layer 311 covering an exposed surface of the dielectric layer 314 and sidewalls of the partition wall 315 therewith.
The substrate 301 in the rear substrate 352 is comprised of a transparent substrate in the display cell illustrated in FIG. 1, however, it is not always necessary for the substrate 301 to be a transparent substrate.
The partition wall 315 defines a discharge gas space and a plurality of display cells (pixels) 308.
Viewing perpendicularly to a surface of the substrate 301, the partition wall 315 is grid-shaped. Specifically, the partition wall 315 is comprised of a vertical partition wall 315a extending in parallel with the data electrodes 317, and a horizontal partition wall 315b extending perpendicularly to the vertical partition wall 315a. 
The vertical and horizontal partition walls 315a and 315b are almost equal in height to each other. A height from a surface of the substrate 301 to a summit of the partition wall 315, that is, a total thickness of the dielectric layer 314 and the partition wall 315 is 120 micrometers, for instance.
Each of the display cells 308 is filled with discharge gas composed of noble gas such as helium, neon or xenon singly or in combination.
The phosphor layer 311 receives ultra-violet rays generated due to discharges of discharge gas, and thus, emits a visible light 310.
An area between the front substrate 351 and the rear substrate 352 is comprised of a centrally located display area in which images are displayed, and a non-display area located around the display area. A partition wall formed in a non-display area is called a dummy partition wall, which assists a partition wall to be uniformly formed in a display area during fabrication of a plasma display panel, and prevents contaminants from entering a display area for protection of a display area after fabrication of a plasma display panel. A dummy partition wall is formed generally by one or two rows.
FIGS. 2A to 4B show respective step in a method of fabricating the conventional plasma display panel illustrated in FIG. 1. FIGS. 2A, 3A and 4A are plan views of the rear substrate 352, and FIGS. 2B, 3B and 4B are cross-sectional views taken along the lines 2B, 3B and 4B in FIGS. 2A, 3A and 4A, respectively.
Hereinbelow is explained a method of fabricating the conventional plasma display panel with reference to FIGS. 2A to 4B.
With reference to FIG. 1, the scanning electrodes 303 and the common electrodes 304 are formed on the substrate 302 such that they are alternately arranged and extend in parallel with each other.
Then, the trace electrodes 305 and 306 are formed on the scanning and common electrodes 303 and 304, respectively.
Then, the dielectric layer 312 is formed on the substrate 302 such that the dielectric layer 312 covers the scanning and common electrodes 303 and 304 and the trace electrodes 305 and 306 therewith.
Then, the protection layer 313 composed of MgO is formed on the dielectric layer 312.
Thus, there is fabricated the front substrate 351.
With reference to FIGS. 2A and 2B, a plurality of the data electrodes 307 is formed on the substrate 301.
Then, as illustrated in FIGS. 3A and 3B, the dielectric layer 314 is formed on the substrate 301 such that the dielectric layer 314 covers the data electrodes 307 therewith.
Then, as illustrated in FIGS. 4A and 4B, the partition wall 315 is formed on the dielectric layer 314.
The partition wall 315 can be formed by sand blasting or printing, for instance. The partition wall 315 is formed as follows in the case that the partition wall 315 is formed by sand blasting.
First, filler, glass powder, binder and solvent are mixed to thereby have partition wall paste.
Then, the partition wall paste is coated on the dielectric layer 314. Then, the solvent in the paste is evaporated to thereby form a partition wall paste layer (not illustrated).
Then, a dry film (not illustrated) is adhered onto a surface of the partition wall paste layer, and then, the dry film is patterned.
Then, sand blasting is carried out to the partition wall paste layer with the patterned dry film being used as a mask. As a result, a portion of the partition wall paste layer not covered with the dry film is selectively removed.
Then, the dry film is removed, and the partition wall paste layer is baked. As a result, the binder in the partition wall paste layer is evaporated, and the glass powder is fused and re-cured. Thus, there is formed the partition wall 315 composed of filler and glass.
The partition wall 315 is formed in a grid such that the vertical and horizontal partition walls 315a and 315b are almost equal in height to each other.
Then, as illustrated in FIG. 1, the phosphor layer 311 is formed on an exposed surface of the dielectric layer 314 and sidewalls of the partition wall 315.
Then, the substrates 301 and 302 are aligned with each other such that the protection layer 313 makes contact with the partition wall 315 and that the data electrodes 307 extend perpendicularly to the scanning and common electrodes 303 and 304.
Then, the substrates 301 and 302 aligned with each other are thermally annealed, resulting in that the substrates 301 and 302 are fused at their ends to each other through flits. Thus, a space surrounded by a sealing layer (not illustrated) comprised of the substrates 301 and 302 and the flits is gas-tightly sealed.
Then, the space is exhausted, and thereafter, discharge gas is introduced into the space.
Thus, there is completed the plasma display panel illustrated in FIG. 1.
However, the above-mentioned conventional plasma display panel is accompanied with a problem of poor quality in displaying images which is caused by contraction of a partition wall paste layer generated during being baked. Hereinbelow is explained the problem of contraction of a partition wall paste layer.
The above-mentioned poor quality in displaying images is grouped into two types.
The first type poor quality is caused by that the vertical partition wall 315a is partially raised during the partition wall paste layer is being baked. The first type poor quality is caused because the vertical partition wall 315a is longer and thinner than the horizontal partition wall 315b. 
Since the vertical partition wall 315a is longer and thinner than the horizontal partition wall 315b, the vertical partition wall 315a and the horizontal partition wall 315b are different from each other with respect to contraction generated during the partition wall 315 is being baked, and hence, the vertical partition wall 315a is partially raised to thereby become higher than the horizontal partition wall 315b. 
As a result, when the substrates 301 and 302 are aligned to each other, a raised portion of the vertical partition wall 315a is compressed by the protection layer 313, and resultingly, the vertical partition wall 315a is often broken. If the vertical partition wall 315a is broken, a portion of the phosphor layer 311 formed on the vertical partition wall 315a itself and sidewalls of the vertical partition wall 315a is scattered into the display cell 308, and resultingly, adheres to the scanning electrode 303 and/or the common electrode 304. This results in that the display cell 308 does not properly operate, that is, the display cell 308 is kept to emit a light regardless of a drive signal or does not emit a light at all.
The second type poor quality is caused by that the vertical and horizontal partition walls 315a and 315b are contracted during the partition wall 315 is being baked, and resultingly, opposite ends of the vertical and horizontal partition walls 315a and 315b in a length-wise direction are deformed to be higher than centers of them.
FIG. 5A is a cross-sectional view illustrating the partition wall 315 before baked, FIG. 5B is a cross-sectional view illustrating the partition wall 315 after baked, and FIG. 5C is a cross-sectional view illustrating the substrates 301 and 302 aligned to each other. For simplification, parts other than the substrates 301 and 302 and the partition wall 315 are omitted in FIGS. 5A to 5C.
As illustrated in FIG. 5A, the partition wall 315 before baked has a uniform height.
However, as illustrated in FIG. 5B, the partition wall 315 is contracted during being baked, and resultingly, opposite ends 315c are raised relative to a central portion 315d. 
As illustrated in FIG. 5C, the substrates 301 and 302 are aligned to each other, and then, a discharge gas space is exhausted. The substrates 301 and 302 are bent due to atmospheric pressure. However, the substrates 301 and 302 are bent in a different curvature from the partition wall 315, and accordingly, gaps 316 are formed between the partition wall 315 and the substrate 302 in the vicinity of the ends 315c. 
As a result, a display cell 308 including the gaps 316 would have an increased volume, and hence, a voltage necessary for generating writing discharge in the display cell 308 would be raised. Thus, writing discharge would not be generated by an ordinary drive voltage in the display cell 308, resulting in writing defectiveness. Thus, the plasma display panel would have a problem of display defectiveness.
There have been suggested solutions to the second type poor quality.
For instance, Japanese Patent Application Publication No. 2001-319580 has suggested a plasma display panel in which a dielectric layer is not formed in a non-display area on a rear substrate, and a partition wall is formed directly on the rear substrate in order to prevent the above-mentioned second type poor quality. This ensures that a partition wall located in a non-display area is lower in height than a partition wall located in a display area. Hence, even if a partition wall is contracted, and accordingly, opposite ends thereof in a length-wise direction become higher than a central area, it would be possible to prevent formation of gaps between the partition wall and a front substrate.
In contrast to the second type poor quality, the first type poor quality is not well recognized, and accordingly, solutions are not much suggested.
For instance, the plasma display panel suggested in the above-mentioned Japanese Patent Application Publication No. 2001-319580 prevents the second type poor quality, but cannot prevent the first type poor quality.
Japanese Patent Application Publication No. 2000-340123 has suggested a plasma display panel which includes an improved horizontal partition wall in order to prevent the first type poor quality.
FIG. 6 is a plan view of a partition wall in the plasma display panel suggested in Japanese Patent Application Publication No. 2000-340123.
As illustrated in FIG. 6, the partition wall is comprised of a plurality of horizontal partition walls 315A horizontally extending, and a plurality of vertical partition walls 315B extending vertically only between adjacent horizontal partition walls 315A.
Each of the horizontal partition walls 315A is designed to have extensions 315C extending from opposite ends thereof. Even if the horizontal partition walls 315A is raised at its opposite ends due to the contraction, such a raise is concentrated to the extensions 315C. Front and rear substrates are joined to each other between the extensions 315C formed at opposite ends of the horizontal partition wall 315A. Accordingly, front and rear substrates can be joined to each other with a constant gap being kept therebetween without being influenced by the raised extensions 315C.
Japanese Patent Application Publication No. 11-339668 has suggested a plasma display panel including a partition wall having opposite tapered ends 315D, as illustrated in FIG. 7, to prevent formation of a raise portion caused by contraction.
The plasma display panel suggested in Japanese Patent Application Publication No. 2000-340123 makes it possible for front and rear substrates to join to each other with a constant gap being kept therebetween. However, since the extensions 315C are raised, if the front and rear substrates are misaligned even slightly, the front substrate aligns with the raised extensions 315C, resulting in that it would not be possible to keep a constant gap between the front and rear substrates.
Accordingly, it is necessary to align the front and rear substrates to each other highly accurately before they join to each other. This causes an additional problem that steps of fabricating a plasma display panel are unavoidably complicated.
The partition wall suggested in Japanese Patent Application Publication No. 11-339668 is formed by physically grinding, punching or a process of half-exposing a partition wall to a light.
If the tapered ends 315D are formed by grinding, there are newly caused problems that a grinding step has to be additionally carried out, and chips are generated in a grinding step.
If the tapered ends 315D are formed by punching or half-exposing process, there are newly caused problems that an equipment for doing so has to be newly prepared, and hence, punching or half-exposing process cannot be applied to a conventional method of forming a partition wall by sand blasting.